JP2000012899A - Manufacture of nitride semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nitride semiconductor device which is superior in uniformity of light characteristics and reliability. SOLUTION: A method of forming an electrode on a nitride semiconductor comprises a first process, where a metal 211 is formed on a nitride semiconductor 200 to be processed into an electrode, a second process where the metal 211 is thermally treated in an oxygen atmosphere, to come into ohmic contact with the nitride semiconductor 200, a third process, where a metal oxide 212 formed on the surface of the metal 211 is removed for the formation of a transparent electrode 201, and a fourth process where a wire bonding pad 202 is formed on the transparent electrode 201.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a nitride semiconductor device having excellent uniformity of optical characteristics and excellent reliability.

[0002]

2. Description of the Related Art Today, nitride semiconductors (In _x Ga _Y Al
Various optical semiconductor elements using _1-XYN , 0 ≦ X, 0 ≦ Y, 0 ≦ X + Y ≦ 1) have been developed. A nitride semiconductor has a wide band gap energy. Therefore, it can be used as a light emitting element capable of emitting red light from ultraviolet light as an LED, an LD (laser diode), or the like. Similarly, various optical semiconductor elements such as a solar cell having high electromotive force and an optical sensor having excellent heat resistance can be formed as the light receiving element.

FIG. 3 is a schematic cross-sectional view showing the basic structure of an LED as a light emitting device using such a nitride semiconductor. The LED chip 300 shown in FIG.
Buffer layer 307 formed at a low temperature on n, n-type GaN
N-type contact layer 306, a light-emitting layer 305 made of InGaN of a thin film which is assumed to produce a quantum effect, p-type G
A cladding layer 304 made of aAlN, a p-type contact layer 303 made of p-type GaN, and translucent p-type electrodes 301 and 311 are provided on almost the entire surface of the p-type contact layer. On a part of the translucent p-type electrode, a p-type pad electrode 302 for wire bonding is formed. The sapphire substrate 308 is insulative and forms p-type and n-type electrodes on the semiconductor layer side.
Part of the surface is partially etched away. Next, a step of forming an electrode on the nitride semiconductor will be described.

A p-type electrode is formed on a p-type contact layer 303 by using a metal laminate as a Ni / A layer by sputtering.
u can be formed. The p-type electrodes 301 and 311 function to uniformly supply current to the nitride semiconductor and emit light to the outside via the p-type electrode. It is relatively difficult to lower the resistance of a p-type nitride semiconductor. Therefore, it is preferable that the p-type electrodes 301 and 311 are formed on substantially the entire surface of the p-type contact layer 303. In addition, it is desirable to select a thin metal or an electrode having a high light-transmitting property for desired light emission emitted from the nitride semiconductor.

As described above, since the translucent p-type electrode is a thin film, a bonding wire for wire-bonding a part of the p-type electrodes 301 and 311 so as not to peel off in a later process. A p-type pad electrode 302 is formed. The p-type pad electrode 302 is a p-type electrode 301,
It can be formed using a sputtering method in the same manner as 311. On the other hand, the n-type electrode 309 is deposited on a thick film to function as an n-type pad electrode. In addition, in order to form various shapes of electrodes, a desired shape can be obtained by using a resist formed in advance.

[0008] Ohmic contact is difficult to obtain only by laminating a metal serving as an electrode. Therefore, a nitride semiconductor is heat-treated in an oxygen atmosphere. Thereby, ohmic contact between each electrode and the nitride semiconductor can be suitably achieved. In addition,
A p-type electrode formed by heat treatment in an oxygen atmosphere has a stacked structure of a layer 301 made of gold and a layer 311 made of nickel oxide. A light emitting diode capable of emitting light can be obtained by sealing the LED chip with a resin (not shown) and injecting current through a wire.

[0007]

However, when the nitride semiconductor device having the above-described structure is formed, a mottled pattern is partially formed on the surface of the light-transmitting electrode even if heat treatment for obtaining ohmic contact is performed. There is something. When a nitride semiconductor device is formed using such mottled electrodes, various inconveniences such as poor light emission in the light emitting device, noise in the light receiving device, and an increase in driving voltage occur. As a result, the yield is greatly reduced. At the present time when higher reliability, optical characteristics, and a higher yield are desired, they are not sufficient, and further improvements are required.

As a result of various experiments, the present inventor has found that uniformity of optical characteristics and reliability can be improved by forming an electrode formed on a nitride semiconductor according to a specific process. It came to accomplish.

Although the reason for the improvement of the characteristics according to the present invention is not clear, it is considered that the heat treatment of the pad electrode formed on the light-transmitting electrode in an oxygen atmosphere is greatly related to the mottled pattern formed on the electrode. That is, it is considered that when the heat treatment for ohmic contact between the nitride semiconductor and the metal serving as the light-transmitting electrode is performed after the pad electrode is formed, heat conduction becomes non-uniform due to the pad electrode, and thermal uniformity may be impaired. . Therefore, in the electrode formed on the surface of the nitride semiconductor, a non-alloyed portion is partially formed (uneven annealing), so that uneven light emission and uneven characteristics occur, thereby lowering the yield. In particular, it can be considered that the translucent electrode appears remarkably because it is much thinner than the pad electrode.

According to the present invention, a heat treatment is performed in an oxygen atmosphere in advance before the pad electrode is formed, so that the ohmic contact between the light-transmitting electrode and the nitride semiconductor is made uniform. On the other hand,
In an oxygen atmosphere, the heat treatment allows the entire surface of the light-transmitting electrode to be formed uniformly and to achieve an appropriate ohmic contact, but a metal oxide is formed on the entire surface of the light-transmitting electrode. Forming a pad electrode on a metal oxide not only increases the resistance, but also decreases the adhesion. Therefore, a portion where the surface of the translucent electrode is oxidized is removed. According to the present invention, a pad electrode can be firmly formed on a light-transmissive electrode that has been uniformly heat-treated, so that the characteristics of the nitride semiconductor element can be stabilized and the yield can be improved.

[0011]

SUMMARY OF THE INVENTION The present invention comprises a step of forming a metal constituting an electrode on a nitride semiconductor, a step of heat treatment in an oxygen atmosphere to obtain ohmic contact between the metal and the nitride semiconductor, Forming a light-transmitting electrode by removing a metal oxide formed on the light-transmitting electrode; and forming a pad electrode for wire bonding on the light-transmitting electrode. . Thereby, ohmic contact and alloying can be favorably achieved even during the heat treatment of the electrode and the nitride semiconductor element. In addition, the adhesion between the translucent electrode and the pad electrode can be improved.

A method according to a second aspect of the present invention includes a step of removing a metal oxide with an acid. As a result, the metal oxide can be suitably removed without damaging the heat-treated metal. Further, the adhesiveness with an electrode formed thereon can be improved.

In the method according to the third aspect of the present invention, the metal layer may be formed of nickel (Ni), chromium (Cr), iron (Fe), copper (Cu), aluminum (Al), silver (Ag), manganese ( A metal layer containing at least one element selected from the group consisting of Mg), tantalum (Ta), and vanadium (V) and a metal containing gold (Au) formed on the metal layer. is there. In particular, even during heat treatment of a plurality of metals, good ohmic contact with the nitride semiconductor can be obtained while preventing alloying unevenness between metal layers.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. 1 and 2 as an optical semiconductor device using a nitride semiconductor. FIG. 1 shows a schematic sectional view of the LED,
FIG. 2 shows each step of forming the LED of FIG.

The LED 100 was formed by MOCVD using the C surface of the cleaned sapphire substrate 108 as a main surface. 0.05μ on a 2-inch diameter sapphire substrate
A buffer layer 107 in which m GaN is formed at a low temperature is formed. On the buffer layer 107, an Si-doped Ga having a thickness of about 4 μm is sequentially formed as an n-type contact layer and a cladding layer.
N106, InGaN 105 having a thickness of about 3 nm acting as a light emitting layer and producing a quantum effect, and Mg-doped AlGaN 10 having a thickness of about 200 nm acting as a p-type cladding layer.
4. Mg-doped GaN 103 having a thickness of 150 nm was formed as a p-type contact layer.

More specifically, a single crystal sapphire substrate is fixed on a heater in a MOCVD apparatus with the c-plane cleaned by organic cleaning and heat treatment as a main surface. Next, the sapphire substrate is baked at a heater temperature of 1100 ° C. while flowing hydrogen H ₂ (hydrogen) gas at normal pressure. Next, the heater temperature is reduced to 500 ° C. Thereafter, TMG (trimethyl gallium) gas, NH ₃ (nitrogen) gas as a source gas, and hydrogen gas as a carrier gas flow. Thus, a GaN buffer layer was formed.

The supply of the source gas is stopped, only the carrier gas is allowed to flow, the temperature is set to 1050 ° C., and again TMG gas and nitrogen gas as the source gas, SiH ₄ (silane) gas as the impurity gas, and hydrogen gas as the carrier gas. An n-type GaN was formed.

The supply of the source gas and the impurity gas is stopped, and only the carrier gas is allowed to flow to lower the temperature to 950.degree.
Thereafter, a TMI (trimethyl indium) gas, a TMG gas, a nitrogen gas, a silane gas as an impurity gas, and a hydrogen gas as a carrier gas are flowed as source gases, and a single quantum well structure having a thickness of 3 nm serving as a light emitting layer is formed.
nGaN was deposited.

Subsequently, the temperature was maintained at 800 ° C., and TMG gas and TMA (trimethylaluminum) were used as raw material gases.
A gas, a nitrogen gas, Cp ₂ Mg (cyclopentadienyl magnesium) as an impurity gas and a hydrogen gas as a carrier gas were flown to form a Mg-doped p-type AlGaN serving as a p-type cladding layer.

In order to form a p-type contact layer, the temperature is again raised to 1050 ° C. while stopping the introduction of the source gas and the impurity gas and flowing only the carrier gas. TMG gas and nitrogen gas as source gases, cyclopentadienyl magnesium as an impurity gas, and hydrogen gas as a carrier gas were flown, and Mg-doped p-type GaN serving as a p-type contact layer was formed.

SiO ₂ was formed at 8000 ° on the surface of the p-type nitride semiconductor by the CVD method. After forming SiO ₂ , a photoresist is uniformly applied, and an exposure process is performed.
The photoresist was partially removed to expose a nitride semiconductor surface on which an n-type electrode was formed through an etching process. SiO ₂ using the remaining photoresist as a mask
Is formed in a desired shape. Next, the nitride semiconductor was partially etched using a chlorine-containing gas by reactive ion etching using SiO ₂ as a mask. As a result, the nitride semiconductor on the sapphire substrate is separated into islands, and the surface of the p-type contact layer and the surface of the n-type contact layer are exposed on the same plane side. After removing the mask, electrodes were formed in the following steps as shown in FIG. FIG. 2A is a schematic cross-sectional view of the nitride semiconductor 200 obtained in the above steps.

Next, the step of forming a metal layer is shown in FIG.
Shown in Specifically, a resist is formed on the nitride semiconductor except for the surface on which the light-transmitting electrode is to be formed, by applying a photoresist, exposing, and etching. This is arranged on one electrode in the sputtering apparatus.
In addition, Ni is disposed on the other electrode as a target. By applying a voltage while introducing Ar gas, Ni was deposited at 100 ° on the p-type nitride semiconductor.
The target was changed from Ni to Au, and Au was placed on Ni for 50
0 ° deposited. The metal laminated film 211 formed in the order of Ni / Au was formed on one surface of the p-type nitride semiconductor by using the lift-off method.

In order to form a metal layer which will later become a light-transmitting electrode on the nitride semiconductor, a sputtering method,
It can be formed using various methods such as a vacuum evaporation method. In order to obtain a desired shape of the light-transmitting electrode, various shapes such as a rectangular shape can be obtained by using a resist. Al, In, Ni, Co, Ag, Au, P as metals capable of ohmic contact with the nitride semiconductor
Preferable examples include t, Ir, Pd, Rh and alloys thereof. The light-transmitting electrode formed on the nitride semiconductor can have various shapes depending on a light-emitting diode, a semiconductor laser, a solar cell, an optical sensor, or the like.

Next, the step of taking ohmic contact is shown in FIG.
This will be described with reference to FIG. With the above-described metal layer 211 formed, a nitride semiconductor was placed in a heating furnace, and heat treatment was performed at a temperature of 400 ° C. in an oxygen atmosphere. After cooling, the nitride semiconductor wafer was taken out and the surface was observed. As a result, a uniform translucent film was formed on the nitride semiconductor wafer. The heat treatment reversed the stacking order, with the Ni element being on the electrode surface side and the Au element being an alloy being on the nitride semiconductor side. Further, Ni on the surface side was oxidized. That is,
A metal layer 201 made of a gold element on the nitride semiconductor side and a metal oxide layer 212 made of nickel oxide are formed.

In order to make ohmic contact between the metal serving as the light-transmitting electrode and the nitride semiconductor, heat treatment can be performed in an inert atmosphere, a hydrogen atmosphere or an oxygen atmosphere. However, in the present invention, the heat treatment is performed in an oxygen atmosphere in order to facilitate ohmic contact with the nitride semiconductor. According to the present invention, the heat treatment is performed in a state where a metal is formed on the surface of the nitride semiconductor and a barrier that causes non-uniform heat conduction is not formed. Thereby, heat treatment can be performed uniformly without heat unevenness on the metal layer. Therefore, it is considered that uneven alloying during the heat treatment is not formed, and the formation of a mottled pattern on the metal is suppressed as much as possible.

In the heat treatment, various heating furnaces can be used as long as heat can be uniformly applied to the semiconductor wafer. The heat applied to the nitride semiconductor can be selected variously depending on the stacked metal or nitride semiconductor. The temperature is preferably 300 ° C. or more and 1100 ° C. or less in order to make ohmic contact and suppress decomposition of the nitride semiconductor. In particular, although it is difficult to make a nitride semiconductor p-type only by containing a p-type impurity, resistance can be reduced by heat treatment. Such a resistance lowering process can also be used for ohmic contact between the electrode and the nitride semiconductor. In this case, the heat treatment temperature of the nitride semiconductor is more preferably 400 ° C. or more and 900 ° C. or less.

Subsequently, a step of removing the metal oxide is shown in FIG.
It is shown in (D). In order to remove the nickel oxide 212 formed on the surface, the nitride semiconductor wafer is immersed in hydrochloric acid stirred at room temperature to remove nickel oxide 2 formed on the surface of the metal.
12 was removed.

When heat treatment is performed in an oxygen atmosphere,
As described above, the surface of the formed translucent electrode is oxidized although the ohmic contact between the nitride semiconductor and the electrode tends to be easily obtained. Such oxidation can enhance the translucency of the metal. However, when a pad electrode for wire bonding is further formed on the metal oxide, the pad electrode on the metal oxide tends to be easily peeled off by the force at the time of wire bonding, and the reliability tends to be low. Similarly, after the formation of the nitride semiconductor element, the semiconductor element or the like may be molded and protected using a resin or the like. The molded resin or the like can protect the nitride semiconductor element from external force from external environment, moisture, dust, and the like. But,
In some cases, a force is locally applied to the pad electrode or the like due to thermal expansion or thermal contraction due to thermal shock or the like, and peeling tends to occur at the interface between the translucent electrode and the pad electrode.

According to the present invention, the adhesion between the translucent electrode and the pad electrode can be improved by removing the metal oxide from the surface of the translucent electrode that has been uniformly heat-treated. As a method of removing the metal oxide, various methods such as etching with an acid can be used in addition to mechanical removal by sputtering or polishing.

FIG. 2E shows the next step of forming a pad electrode. A photoresist is formed on the surface on which the p-type electrode is formed through application of a photoresist, an exposure process, and an etching process, except for the surface on which the p-type pad electrode is to be formed. The nitride semiconductor wafer thus formed was placed in the sputtering device again. Au was deposited on the translucent electrode by applying a voltage while introducing Ar gas while using Au as a target to deposit 7000 °.
Change Ni from Au to Ni
(4) The p-type pad electrode 202 was formed by deposition.

The pad electrode 102 is at least partially formed on the translucent electrode 101. Pad electrode 102
Is to prevent the light-transmitting electrode and the like from being damaged by performing wire bonding or the like directly on the light-transmitting electrode 101. Therefore, it is desirable to select a metal having good adhesion to the translucent electrode and excellent electric conductivity. As a specific material of the pad electrode, various metals can be selected in consideration of adhesion to the translucent electrode and conductivity.
Metals and alloys such as u, Ag, Cu, Ni, Co, W, Ti, Sn, Pb and In can be suitably selected.

The pad electrode can be formed by using a vacuum evaporation apparatus or various film forming apparatuses in addition to the above.

Finally, FIG. 2F shows a film forming process for forming an n-type electrode and an n-type pad electrode. As an n-type electrode and an n-type pad electrode, a desired mask is formed in the same manner as the p-type pad electrode except that the target is W or Al in order to form a W / Al laminate, and the n-type electrode 109 is formed by sputtering. Was formed. After the mask is removed, W is stacked on the n-type contact layer as an n-type electrode by 200 ° and Al is stacked by 2000 °.

The nitride semiconductor wafer is separated into individual nitride semiconductors by using a dicing saw and a scriber. 500 of the separated LED chips were examined. The surface of each LED chip was a uniform transparent film. Further, an LED chip was fixed on the lead electrode, and a p-type pad electrode was ball-bonded using a gold wire having a diameter of 30 μm, and wire bonding was performed on the lead electrode as stitch bonding. Similarly, the n-type electrode and another lead electrode were wire-bonded (not shown). When a current is applied between the lead electrodes, all emit light uniformly.

Next, the adhesion strength between the p-type electrode and the p-type pad electrode was examined. This was compared with an LED chip formed in exactly the same manner except that nickel oxide was not removed. The wire connecting the p-type pad electrode and the lead electrode was pulled up with a wire while gradually increasing the tensile strength. Even if 10% of the LED chip formed for comparison peeled off at the interface between the p-type electrode and the p-type pad electrode, the LED chip of the present invention was not damaged at all. When the weight was further increased, the number of peeled-off LED chips for comparison was further increased. Further, although the breaking of the gold wire started to occur in all the LED chips, none of the LED chips of the present invention peeled off at the interface between the p-type electrode and the pad electrode. The same characteristics are exhibited when the above-described metal element capable of ohmic contact is added to nickel instead of nickel laminated for a p-type electrode.

[0036]

According to the structure of the nitride semiconductor device of the present invention, it is possible to increase the uniformity of the optical characteristics at the time of light emission or light reception and to improve the adhesion between the light transmitting electrode and the pad electrode. it can.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view showing a nitride semiconductor device formed by a process of the present invention.

FIG. 2 is a schematic view showing a step of forming a nitride semiconductor device of the present invention.

FIG. 3 is a schematic sectional view and a plan view showing a nitride semiconductor device shown for comparison with the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 100 ... Light-emitting element 101 ... Translucent p-type electrode 102 ... p-type pad electrode 103 ... p-type contact layer 104 ... p-type cladding layer 105 ... active layer 106 ... · N-type contact layer 107 ··· buffer layer 108 ··· sapphire substrate 109 ··· n-type electrode 200 ··· nitride semiconductor element before electrode formation 201 ··· translucent electrode 202 ··· -P-type pad electrode 203 ... n-type pad electrode 211 ... metal constituting a translucent electrode 212 ... metal oxide 300 ... light emitting element 301 ... translucent p-type electrode 302 ... p-type pad electrode 303 ... p-type contact layer 304 ... p-type cladding layer 305 ... active layer 306 ... n-type contact layer 307 ... buffer layer 308 ... sapphire substrate 311 ... Metal oxides

Claims (3)

[Claims] A step of forming a metal constituting an electrode on the nitride semiconductor; a step of performing heat treatment in an oxygen atmosphere to obtain ohmic contact between the metal and the nitride semiconductor; and a step of forming a metal on the surface of the metal. A method for manufacturing a nitride semiconductor device, comprising: a step of forming a translucent electrode by removing an oxide; and a step of forming a pad electrode for wire bonding on the translucent electrode. 2. The method according to claim 1, wherein said metal oxide is removed with an acid.
3. The method for manufacturing a nitride semiconductor device according to item 1. 3. The method according to claim 1, wherein the metal is nickel (Ni), chromium (Cr), iron (Fe), copper (Cu), aluminum (A).
l), silver (Ag), manganese (Mg), tantalum (T
a), a metal layer containing at least one element selected from vanadium (V), and gold (A) formed on the metal layer.
2. The method for producing a nitride semiconductor device according to claim 1, wherein the laminate is at least two or more kinds of laminates with a metal containing u).